Recently, a demand for polymer products has rapidly increased worldwide, particularly, in China. Therefore, production and stable supply of light olefins as raw materials for producing these products has become very important. Among those light olefins, demand for and value of n-butene and 1,3-butadiene which serve as a raw material for various synthetic rubber and copolymer products are increasing currently, and methods for producing them include three methods such as naphtha cracking, direct dehydrogenation of n-butane or n-butene, and oxidative dehydrogenation of n-butane or n-butene.
In recent years, as for a naphtha cracking process which contributes more than about 90% of normal-butene and 1,3-butadiene supply, new establishment or expansion plans have been reported one after another, and actually, the factories have been in operation. However, although such new establishment or expansion plans of naphtha cracking center seem to be sufficient for a recently increasing demand for normal-butene and 1,3-butadiene as raw materials for producing synthetic rubber and polyethylene products, a look at naphtha crackers being established exhibits that they are not. For example, a gas cracker being established in the Middle East and using a low-priced gas as a raw material is intended to produce olefins such as ethylene, propylene, etc, and thus has a low yield with respect to C4 mixtures. Therefore, new establishment or expansion of naphtha cracking center only for the purpose of increasing the production of n-butene and 1,3-butadiene is hardly considered, and if so, it would cause further problems of surplus production of other basic petrochemical feedstocks other than n-butene and 1,3-butadiene. Moreover, with an increasing demand for ethylene and propylene in the recent petrochemical market, new establishment and operation regarding a naphtha cracking process tends to be rather focused to increase in production yield of ethylene and propylene. In addition to that, with the continuous price increase in naphtha raw materials for C4 production, a naphtha cracking process is modified as a process using light hydrocarbons such as ethane, propane, etc. as a raw material which can result in high production yield for basic petrochemical feedstock such as ethylene, propylene and the like, although its yield for C4 mixtures is low, and thus the proportion of a process for obtaining C4 mixture in the naphtha cracking process is relatively reduced. Therefore, it is getting more difficult to obtain C4 mixtures, particularly normal-butene and 1,3-butadiene, by the naphtha cracking process.
As the foregoing description, although n-butene and 1,3-butadiene supply majorly depend on a naphtha cracking process, based on the many reasons as above, the naphtha cracking process cannot be an alternative way to resolve the imbalance between supply and demand, caused by recent increased demand in n-butene and 1,3-butadiene. In this circumstance, a dehydrogenation reaction in which hydrogens are removed from n-butane thus obtaining n-butene and n-butene, is attracting great attention as an effective process for responding to the increased demand for n-butene and 1,3-butadiene, and thus many studies regarding that are being made [Non-Patent Documents 1 to 4].
The dehydrogenation reaction of n-butane can be classified into direct dehydrogenation and oxidate dehydrogenation, wherein the direct dehydrogenation reaction of n-butane is highly exothermic and thus a thermodynamically disadvantageous reaction since hydrogen should be directly detached from a chemically stable n-butane as well as requires great energy consumption to satisfy the high-temperature reaction condition. For carrying out direct dehydrogenation, used are precious metal catalysts such as platinum or palladium. However, such precious metal catalysts have a problem of requiring a reactivation process owing to their short lifetime in most cases, therefore the direct dehydrogenation is not a suitable commercial process for producing 1,3-butadiene [Patent Documents 1 to 2].
On the contrary, unlike the direct dehydrogenation, the oxidative dehydrogenation of n-butane is the reaction wherein n-butane and oxygen reacts to produce n-butene and water, or 1,3-butadiene and water, and thus, compared to the direct dehydrogenation, it is thermodynamically advantageous due to the generation of stable water, and since an endothermic reaction turns to an exothermic reaction with the generation of water, and rapid temperature changes in catalyst layer which can caused by the heat from the catalyst reaction can be prevented by water generated after the reaction. By such reasons, the oxidative dehydrogenation process of n-butane can be operated under process conditions more advantageous than those of the direct dehydrogenation process. Therefore, when a catalyst process for producing n-butene and 1,3-butadiene with high efficiency is developed, this process can be used as an effective alternative to prior processes to produce n-butene and 1,3-butadiene through an independent energy-saving process.
As described above, although the oxidative dehydrogenation of n-butane has many advantages over the direct dehydrogenation of n-butane in many ways such as a thermodynamic aspect which makes possible to produce n-butene and 1,3-butadiene with a high yield, under mild reaction conditions, it has a drawback that many side reactions such as highly oxidative reactions which involve generation of carbon monoxide or carbon dioxide owing to the use of oxygen as a reactant. Therefore, in order to commercialize the oxidative dehydrogenation of n-butane, the most crucial technical point is to achieve a catalyst with highly increased selectivity to n-butene and 1,3-butadiene by preventing side reactions such as complete-oxidative reactions, while achieving the conversion of n-butane to the maximum.
Although the reaction mechanism of the oxidative dehydrogenation of n-butane has not yet been exactly known, it is reported that, as a first step, hydrogen is detached from n-butane adsorbed to a solid catalyst due to a reaction between metallic active sites and lattice oxygen of the catalyst at the same time when a redox reaction of the catalyst occurs with a loss of the lattice oxygen from the catalyst, and therefore complex oxide catalysts containing transition metal ions which may be in various oxidation states are essential to this oxidative dehydrogenation reaction [Non-Patent Document 5].
So far, cataslyts known to effectively produce n-butene and 1,3-butadiene through oxidative dehydrogenation of n-butane are magnesium orthovanadate catalysts [Non-Patent Documents 4 and 6 to 8, and Patent Documents 3 to 5]; vanadium oxide catalysts [Non-Patent Documents 9 to 10, and Patent Document 6]; pyrophosphate catalysts [Non-Patent Documents 2 and 11]; ferrite catalysts [Non-Patent Document 12 and Patent Document 7] and the like. The characteristic feature shared by the above complex oxide catalysts is the presence of transition metals, which are necessary for transition of electrons between the catalyst and n-butane via the redox reaction of the catalyst as explained above [Non-Patent Document 13]. The catalysts can carry out the oxidative dehydrogenation of n-butane by incorporating metals which can be oxidized and reduced such as, for example, vanadium, iron, nickel and titanium, etc, and among them, particularly, magnesium orthovanadate catalysts which contain vanadium are known to have high activity, based on which it is considered for the redox potential of vanadium metal to be suitable for the oxidative dehydrogenation of n-butane [Non-Patent Documents 6 to 7].
Magnesium orthovanadate catalysts are generally produced to be the form in which the active phase of Mg3(VO4)2 is supported by a separate metal oxide. It is reported that when magnesium orthovanadate catalysts are not supported, the activity is lower than that of supported magnesium orthovanadate. For example, some results of oxidative dehydrogenation of n-butane by using unsupported magnesium orthovanadate catalysts have been reported in conventional patents and literatures, specifically, for example, 11.5% of n-butane conversion rate, 6.7% of dehydrogenation product yield under the conditions of 540° C. and the feed composition ratio of n-butane:oxygen:helium=4:8:88 [Non-Patent Document 8], and 5.7% dehydrogenation product yield under the conditions of 540° C. and the feed composition ratio of n-butane:oxygen:helium=5:10:85 [Non-Patent Document 4]. Further, it was reported that an oxidative dehydrogenation reaction of n-butane under the conditions of 540° C. and the feed composition ratio of n-butane:oxygen=1:2 using an unsupported magnesium orthovanadate catalyst resulted in a n-butane conversion rate of 10.5% and a dehydrogenation product yield of 5.7% [Non-Patent Document 4].
When magnesium orthovanadate catalysts are supported, the activity can be more improved. Specifically, magnesia supported magnesium orthovanadate catalysts obtained by supporting vanadium to excessive amount of magnesia and their excellent activity for the oxidative dehydrogenation of n-butane have been generally reported. Specifically, it was reported that when the oxidative dehydrogenation of n-butane under the conditions of 600° C. and the composition ratio of n-butane:oxygen:nitrogen of 2:1:97 was conducted by using a magnesia-supported magnesium orthovanadate catalyst obtained by mixing magnesium hydroxide with a mixed aqueous solution of ammonium vanadate and ammonia with the ratio of Mg to V of 6:1, it resulted in 30.4% of n-butane conversion rate, 70.6% of dehydrogenation product selectivity and 21.5% of dehydrogenation product yield [Non-Patent Document 1], and when the oxidative dehydrogenation of n-butane under the conditions of 540° C. and the composition ratio of n-butane:oxygen:helium of 5:10:85 was conducted by using a magnesia-supported magnesium orthovanadate catalyst, it resulted in the yield of 22.8% [Non-Patent Document 4]. Further, also reported were the results of 35.4% of n-butane conversion rate and 18.1% of dehydrogenation product yield by the oxidative dehydrogenation of n-butane under the conditions of 550° C. using a magnesia-supported magnesium orthovanadate catalyst under the higher oxygen conditions wherein the feed composition ratio of n-butane:oxygen:helium=5:20:75 [Non-Patent Document 14].
Further reported was a method for using magnesium orthovanadate catalyst which makes possible to increase the activity for the oxidative dehydrogenation of n-butane by mixing additives to magnesia-supported magnesium orthovanadate catalyst so as to obtain products from the dehydrogenation, n-butene and 1,3-butadiene with high yield in the literature of [Non-Patent Document 15], wherein the dehydrogenation was carried out under the conditions of 570° C., a composition ratio of n-butane:oxygen:nitrogen of 4:8:88 by using 25 wt % of a magnesia-supported magnesium orthovanadate catalyst further mixed with titanium oxide and chromium oxide, resulting in 54.0% of n-butane conversion rate and 33.8% of dehydrogenation product yield.
The magnesia-supported magnesium orthovanadate catalyst makes it possible to obtain n-butene and 1,3-butadiene at a very high yield in an oxidative dehydrogenation reaction of n-butane, but oxidation-reduction of the catalyst which should be reversibly carried out in a catalytic reaction is non-reversibly carried out [Non-Patent Document 1], and, thus, the high activity of the magnesia-supported magnesium orthovanadate catalyst is not maintained for a long time. Therefore, it is limited in application as a commercialized process.
With a purpose to overcome the limits of magnesia-supported magnesium orthovanadate catalysts of the prior arts, the present inventors have developed and reported a method of producing a magnesia-zirconia complex carrier-supported magnesium orthovanadate catalyst which is thermally and chemically stale and does not have any problem regarding a decrease in catalytic activity over time in magnesia-supported magnesium orthovanadate catalysts or a low activity in vanadium-based catalysts of the prior arts; and a method for producing n-butene and 1,3-butadiene with a stable and high yield, using the catalyst produced by the above method [Patent Documents 3 to 5]. Specifically, in the previous patents, the present inventors produced a magnesia-zirconia complex carrier for a catalyst for an oxidative dehydrogenation of n-butane by gel-oxalate method, and produced a magnesia-zirconia complex carrier-supported magnesium orthovanadate catalyst by supporting vanadium to the carrier; and thereby established a catalytic process for producing n-butene and 1,3-butadiene with a stable and high yield, using the catalyst.